This invention relates to radiation-emitting panels and display assemblies.
In many applications, it is desired to be able to produce even illumination over a large area, such as when back lighting instruments. Preferably, the illumination is of high intensity with a low power and heat dissipation whilst being compact and of light weight.
Fluorescent lighting, in which light is generated by photo-ionisation of a phosphor layer in a gas-discharge tube, is ideally suited to this, as far as the level of illumination and power dissipation is concerned. Where even illumination is required over a large area, however, it is necessary either to use several fluorescent tubes in parallel with one another or to use a tube that is bent, in an attempt to produce an even distribution of light. In WO 87/04562 there is described a display in which an arrangement of parallel tubes is reproduced in a flat panel by means of walls that divide the panel into separate discharge paths, each having their own electrode. A bent tube arrangement is similarly reproduced by walls defining a circuitous path between two electrodes. It is usually also necessary to use some form of diffuser in front of such arrangements to produce a more even illumination. This does still not produce illumination which is distributed sufficiently evenly for some applications because of the presence of the walls.
WO 87/04562 also describes a flat panel fluorescent device formed by two glass plates coated with phosphor on their facing surfaces. The plates are spaced from one another and sealed around their edges, the space between the plates being evacuated to a low pressure. Electrodes extend along opposite edges inside the space between the plates, so that discharge can be produced between them. The problem with this construction is that, because of the reduced pressure within the device the plates must be relatively thick to be able to withstand the pressure differential across them. This leads to a device which is relatively heavy and bulky.
Proposals for supporting the opposite plates of the panel by internal structures have been described in J58-46568, GB 2217905, EP 0283014 and PCT/GB90/00075.
Although flat panel lamps are generally more robust than those involving fluorescent tubes, they do suffer from the disadvantage that if the lamp does fail, all illumination is lost. By contrast, where the lamp includes several fluorescent tubes, failure of one tube will only result in a reduction in the level of illumination and uneven illumination. Where the lamp is used to illuminate an instrumentation display, this uneven illumination can, however, be so severe as to make the display illegible in parts. In some applications, such as in aircraft instrumentation displays, it is of great importance that the lamp have some redundancy or a back-up be provided so that adequate illumination is provided on failure of the main lamp. The back-up lamp should also produce illumination that is sufficiently even to render the display legible.